Thermogenic hydrocarbons are generated in the subsurface from source rocks rich in organic matter. Following initial deposition, source rocks are buried and subjected to increasing temperature and pressure with increasing burial. Thermogenic hydrocarbons are then generated when the source rocks reach temperatures sufficient for the thermal conversion of organic material to kerogen and then to free liquid and/or gaseous hydrocarbon phases in a process called source rock maturation. Upon generation, the hydrocarbons may subsequently be expulsed from the soured rock and migrated in the subsurface to reservoir rocks (such as sandstones or limestones) that have sufficient porosity, structure, and an adequate seal that make them capable of trapping the hydrocarbon phase(s), allowing hydrocarbons to accumulate.
In contrast to thermogenic hydrocarbons which are generated during source rock maturation processes, biogenic hydrocarbons are generated as byproducts from microbial utilization of buried organic matter in the subsurface. The generation of biogenic hydrocarbons usually occurs early during sediment burial (e.g., primary biogenic gas generation), but can also occur during the degradation of thermogenic hydrocarbons, for example as a byproduct of microbial consumption of thermogenic hydrocarbons (e.g., secondary biogenic gas generation).
Conventional pressure depletion methods that are used to produce oil from subsurface accumulations typically result in only modest recovery factors. Often, approximately 60-80% of the oil in the subsurface remains inaccessible when such conventional pressure depletion production ends. As such, enhanced recovery methods, such as microbial enhanced oil recovery (“MEOR”) techniques, are then used to try to access the residual oil to extend the life of the production field.
Similar challenges exist when producing biogenic gas fields that are present within coal seams or are sorbed to coal (i.e., coalbed methane). In such systems, biogenic gas is generated by the microbial degradation of the coal. However, over time the biogenic gas generation can decrease and the recovery factor can decrease. Similar to MEOR techniques, stimulation of the microbial populations in these coal systems can be undertaken to regenerate the microbial coalbed methane production to form microbial enhanced coalbed methane (“MECoM”) and extend the production life of these types of assets.
MEOR and MECoM techniques typically involve either the external stimulation of in situ indigenous microbial communities or the introduction of exogenous microbial communities (e.g., microbial populations that are introduced to the subsurface through water/fluid injection into the subsurface). The use of microbial stimulation techniques can improve the properties of the crude oil in the formation; modify the wettability in the reservoir (e.g., the microorganisms can mediate changes in the wettability of oil droplets by growing on the droplet and changing the surface of the oil to a less hydrophobic surface); generate biosurfactants that can reduce interfacial tension; make the hydrocarbons more mobile in the subsurface (e.g., the microorganisms can produce lower molecular weight hydrocarbons by enzymatically cleaving larger hydrocarbons into smaller molecules, thereby reducing the crude oil's viscosity); alter the permeability of the formation (e.g., the microorganisms can produce low molecular weight organic acids from the biodegradation of hydrocarbons which can cause rock dissolution); increase formation pressure (e.g., the microorganisms can generate gases, such as carbon dioxide and nitrogen, that can affect the formation pressure); and/or increase the generation of biogenic gas in the formation.
While microbial stimulation techniques can be very useful, it can be difficult to accurately monitor their performance. That is, it can be difficult to obtain an accurate in situ sample of the microbial community to test and evaluate their performance. Further it can be difficult to predict which microbial stimulation techniques will perform the best and how different microbial communities will react and perform under the harsh environmental conditions deep in the subsurface, as it is difficult to replicate such conditions in a laboratory environment.
The environmental conditions (e.g., temperature, pressure, formation water salinity, reservoir lithology, etc.) of reservoir formations and coalbed systems can vary significantly. The structure of microbial communities (e.g., the type of microorganisms that are present in the community and their relative proportion of the total microbial population) are very sensitive to the environmental conditions as different microbial species have different tolerances for temperature, salinity, and nutrient supplies. For example, microbial communities that exist in shallow reservoirs with fresh formation water are likely different from those that prevail at deeper, warmer reservoirs with higher salinity formation water.
Further, in MEOR and MECoM techniques that involve the introduction of nutrients to the formation to stimulate indigenous microorganisms, the process of stimulating the indigenous microbes can be unpredictable. For example, the growth of the microbial community can produce beneficial effects by, for example, dislodging oil entrapped within the formation. However, alternatively, the grown of the microbial community can lead to increased consumption of light oils (e.g., short-chain alkanes) which can make the oil more viscous and, thus, lower the recovery factor.
Thus, there remains a need for methods and techniques to evaluate the efficiency and efficacy of microbial stimulation operations. In particular, there remains a need for methods and techniques for monitoring and evaluating the relative performance of different stimulation operations, for determining whether appropriate stimulation treatments are being employed for a given environment, and for determining whether an optimized frequency of treatments is being utilized.
Background references may include US Patent Application Publication Nos. US 2014/0250999 A1, and US 2014/0288853 A1; PCT Publications WO 2007/008932 A2, WO 2013/071187 A1, WO 2013/071189 A1, WO 2016/043980 A1, WO 2016/043981 A1, WO 2016/043982 A1, WO 2016/126396 A1, and WO 2016/126397 A1; and Bryant et al. (1989) “Review of Microbial Technology for Improving Oil Recovery”, SPE Reservoir Engineering, Vol. 4, pp. 151-154; and Van Hamme et al. (2003) “Recent Advances in Petroleum Microbiology”, Microbiology and Molecular Biology Reviews, Vol. 67, No. 4, pp. 503-549; and Sandrea et al. (2007) “Global Oil Reserves—Recovery Factors Leave Vast Target for EOR Technologies”, Oil & Gas Journal, Vol. 105, pp. 44-47; and D. Hohl et al. (2010) “Energy, Environment and Climate Directorate White Paper”, DCO Energy, Environment, and Climate Workshop, pp. 1-38; and G. Hassanzadeh et al. (2011) “Petroleum System Analysis Using Geochemical Studies, Isotope and 1D Basin Modeling in Hendijan Oil Field, SW Iran”, International Petroleum Technology Conference, 14797, pp. 1-11; and Li et al. (2014) “Microbial Abundance and Community Composition Influence Production Performance in a Low-Temperature Petroleum Reservoir” Environmental Science & Technology, Vol. 48, pp. 5336-5344; and D. A. Stopler (2014) “New Insights Into the Formation and Modification of Carbonate-Bearing Minerals and Methane Gas in Geological Systems Using Multiply Substituted Isotopologues”, California Institute of Technology Thesis, pp. 1-305; and Stopler et al. (2014) “Formation temperatures of thermogenic and biogenic methane”, Science, Vol. 344, pp. 1500-1503; and Stopler et al. (2015) “Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues”, Geochimica et Cosmochimica Acta, Vol. 161, pp. 219-247; and Wang et al. (2015) “Nonequilibrium clumped isotope signals in microbial methane”, Science, Vol. 348, pp. 428-431.